P5 Electrical Circuits

Static Electricity

Build up of static is caused by friction.

When 2 insulating materials are rubbed together, electrons are scraped off one and dumped onto the other. This leaves a positive charge on one and a negative charge on the other e.g. Polythene and acetate roods rubbed with a cloth duster (look at picture on page 50). Which ways the electrons are transferred depends on the 2 materials involved.

Only the electrons move- never the positive charges. The electrons move away= where they have come from is now positively charged.

When 2 insulating materials are rubbed together, a whole load of electrons get bumped together. They try to repel but cant move apart- the patch of charge created is static electricity. (Read bottom of page 50).

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Electric Current

Electric current is a flow of charge around a circuit. Current= amperes (A)

Wires are full of charges that can move.

Electric charge flows in a metal conductor as there are lots of free electrons that are free to move around. Current can't flow in an insulator e.g. plastic as the electrons are fixed.

In a complete circuit, the battery pushes charge through the wires. The charge flows all the way around the circuit and back to the battery. The current is NOT USED UP!

Current depends on the voltage and resistance. Current will only flow through components if there is a voltage across it.

Voltage (Volts, V) is the driving force which pushes the current around.

Resistance (Ohms) is anything in the circuit which slows the flow down.

Increase in the voltage= more current flows

Increase in resistance= less current flows.

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Electric Current Cont.

Current is like a flow of water. The pipes are full of water, the wires are full of charge that is free to move.

Voltage is like the pressure provided by a pump (battery in the circuit) which pushes the water (charge) round without it being used up.

Resistance is any sort of constriction in the flow.

If you turn up the pump (battery), it will provide more pressure (voltage) so the flow will increase. If you put in more constrictions (resistance), the flow will decrease.

Electrons in the circuit flow the opposite way to convectional current.

Current flows from positive to negative (right to left).

Electrons flow from negative to positive (left to right).

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Circuits

Very basic circuits are used for testing components.

Voltmeters are only placed in parallel so it can compare the charge before and after the component by measuring the potential difference (voltage) between the 2 points.

Potential difference tells you how much energy is transferred to or from a unit of charge as it moves between 2 points.

The battery transfers energy to the charge as it passes and components transfer energy away from the charge as it passes e.g. use as light in a lamp.

Fixed resistor Ammeter

Variable resistor Voltmeter

Thermistor Battery

LDR Power supply

Battery

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Resistance

The slope of a voltage-current graph shows the resistance.

The current through a resistor (at constant temperature) is proportional to the voltage. Different resistors= different resistances - steeper the slope, the lower the resistance.

Resistors get hot when current passes through them.

When the electrons move through a resistor they collide with stationary particles in the resistor making the resistor heat up = a change in resistance.

Series Circuits

In series circuits, the different components are connected in a line, end to end (except voltmeters which are always connected in parallel).

If you disconnect one component, the circuit is broken and wont work.

Very few things are now connected in series.

Potential difference is shared between the various components. V= V1 + V2. The voltage around the circuit adds up to the source voltage.

The energy transferred to the charge by the battery is the same as the total energy transferred from the charge to the components.

Current is the same everywhere. A1 = A2 = A3. Its determined by the total P.D. of the cells and the total resistance of the circuit. This means all the components get the same current.

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Series Circuits Cont.

Resistance is the sum of all the individual resistors. R= R1 + R2 + R3. The resistance of 2 or more resistors in series is bigger than the resistance of just 1 of the resistors on its own because the battery has to push charge through all of them.

The bigger the resistance of a component, the bigger its share of the total P.D. as more energy is transferred when the resistance is high.

Battery (cell) voltages add up.

If you connect several cells in series, all the same way, you get a bigger total voltage. So two 1.5V cells in series would supply 3V in total.

*The current through a component depends on its resistance. The current through each component is the same as if the component was the only one in the circuit. The resistance controls how much current and voltage is able to push through it.

All the components have the same P.D across them, so the component with the most resistance has lowest current and the component with the least resistance has the highest current.

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Parallel Circuits

Much more common than series circuits.

Each component is separately connected to the +ve and -ve of the supply. If you remove or disconnect one component, it will hardly affect the others.

Potential difference is the same across all the components. All components get a full source of P.D. V1 = V2 = V3. This means that identical bulbs connected in parallel will be at the same brightness.

The total current is the sum of the current going to each of the different components. A = A1 + A2. In parallel, the current flowing from the battery is shared between the branches. So the total current leaving the battery is equal to the currents in the separate branches. Total current going into a junction = the total current leaving.

The resistance is lower as the current has more than one branch to take- only some current flows along each branch. A current with 2 resistors in parallel will have a lower resistance than a circuit with either of the resistors by themselves- which means the parallel circuit will have a higher current. Total R < R1 and total R < R2. *

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Mains Electricity

Mains electricity is Alternating Current (AC), battery supply is Direct Current (DC).

AC- Constantly changing DC- Current always flows in the same direction.

AC is used for mains as its easy to generate and can be distributed more efficiently.

The UK mains supply is 230V. Its produced by generators using a process called electromagnetic induction.

Moving a magnet in a coil of wire induces a voltage. By moving the magnet it creates a voltage and maybe a current in a conductor- this is electromagnetic induction. As you move the magnet, the magnetic field through the coil changes= this change is a voltage. The way you move the magnet the voltage is induced in the opposite direction (page 58).

AC Generators- in the generator a magnet rotates in a coil of wire. As the magnet turns, the magnetic field through the coil changes- this change in the magnetic field creates a voltage which makes the current flow. When the magnet is turned through 1/2 a turn the direction of the magnetic field through the coil reverses, so the voltage reverses, so the current flows in the opposite direction. If the magnet keeps turning in the same direction = Alternating Current is produced.

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Mains Electricity Cont.

3 Factors affect the size of the induced voltage. To make it bigger, increases one or more of:

The strength of the magnet.

The speed of the movement.

The number of turns on the coil.

To make the induced voltage smaller you reduce one or more of the above.

Moving the magnet faster creates a higher peak voltage, but also a higher frequency.

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Transformers- Mains Electricity

Transformers change AC voltages. They use electromagnetic induction to 'step up' or 'step down' the voltage. They have 2 coils of wire, the primary and the secondary wound around an iron core.

Step up transformers step the voltage up. They have more turns on the secondary coil than the primary coil. Step down transformers step the voltage down. They have more turns on the primary coil than the secondary.

Transformers work by electromagnetic induction. The primary coil produces a magnetic field which stays within the iron core. Because there's alternating current in the primary coil it causes the magnetic field to constantly change direction, this changing magnetic field induces an alternating voltage in the secondary coil (with the same frequency as in the primary).

If you supplied DC to the primary coil, you would get nothing, the magnetic field wouldnt be constantly changing.

The relative number of turns on the 2 coils determines whether the voltage induced in the secondary coil is greater or less than the voltage on the primary coil.

You can use the transformer equation either way up to calculate the output voltage. Make sure you put what you are trying to work out on the top.

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Electrical Energy

Energy is transferred from cells and other sources. Anything that supplies electricity is also supplying energy.

Power is the rate of energy transfer. The power of an appliance tells you how fast it transfers energy from the charge passing through it. Power is measured in Watts (W) or kilowatts (Kw). 1Kw = 1000W

A light bulb converts electrical energy into light energy and heat energy. A power rating of 100W means it transfers 100 joules every second.

Kilowatt-hours (KWh) are units of energy. Energy is usually measured in joules, but it's too tiny amount- so electricity meters record how much energy you use in units of KWh. A kilowatt-Hour is the amount of electrical energy converted by a 1Kw appliance left for 1 hour.

The higher the power rating and the longer you leave the appliance on for, the more energy it transfers and the more it costs. Cost= No.of KWh x Cost per KWh.

An appliance with high power rating transfers a lot of energy in a short time. This energy comes from the current flowing through it. Appliance with a high power rating will draw a large current.

More efficient machines waste less energy as heat or sound most is converted usefully.